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hydromedusae dominate numerically. Adding to the confusion is the fact that many, but not all, of the hydromedusae and scyphomedusae alternate generations from a sedentary, anemone‐like, polyp stage to a swimming medusoid form. The alternation of generations is sometimes termed “metagenesis.” It was the alternation of generations that captured the imagination of famous natural philosophers such as Cuvier, Lamarck, and especially Ernst Haeckel, who made siphonophores one of his favorite subjects for study.

      Siphonophores are unfamiliar to most people not well acquainted with the open ocean because the animals are found predominantly offshore and are very delicate. Therefore, they are difficult to preserve or to view intact after capture. As a consequence, siphonophores have received limited study though they are common predators in the open ocean and especially in the deep sea.

Classification

      History

The sedentary nature of many of the Cnidaria (think anemone, not jellyfish) made their position within the animal kingdom difficult to decipher for the early natural philosophers, who considered them something somewhere between an animal and a plant. Aristotle called them Acalephae or Cnidae (from the Greek: akalephe = nettle; cnidos = thread) (Hyman 1940) and placed them with a variety of other soft‐bodied animals in with the Zoophyta (from the Greek: zoon = animal, phyton = plant). Researchers in the eighteenth century recognized the animal nature of cnidarian polyps, leading the way for natural philosophers of the nineteenth century, Linnaeus, Lamarck, and Cuvier, to place the cnidarians among the animals in their own classification systems, among either the Radiata, recognizing the importance of radial symmetry, or Zoophyta.

      Just as happens today, the natural philosophers of the nineteenth century had differences of opinion on the group relationships. Lamarck’s system included the medusoid Cnidaria and echinoderms (starfish) in his Radiata, with the polypoid cnidarians simply called the Polyps. In 1829, Sars showed that polyps and medusae were different life stages of the same animal, not separate groups. Not quite 20 years later, Leuckart and Frey (1847) separated the two largest radially symmetrical groups, the echinoderms and the cnidarians, into two groups: the coelenterates and the echinoderms. Leuckart coined the term Coelenterata from the Greek words for body cavity (koilos) and intestine (enteron), noting that the only body cavity in the cnidaria was the intestine (Hyman 1940). Leuckart included the sponges and ctenophores within his Coelenterata. It was up to Hatschek (1888) to separate Leuckart’s Coelenterata into the three phyla we recognize today: the Spongiaria (Porifera), the Cnidaria, and the Ctenophora. The term Coelenterata is still extensively used today, most commonly as a synonym for the Cnidaria but sometimes as a way of combining the ctenophores and cnidarians into a single taxon.

      Classification Schemes

      Classification within the Cnidaria is constantly evolving. While there is consensus on what the different groups within the Cnidaria are, there is considerably less agreement on rank and position within the systematic hierarchy. It is important to recognize that molecular methodologies have opened a new and rapidly evolving way of classifying species at all levels of the taxonomic hierarchy. The half‐life of any taxonomic scheme will likely be quite short for some time to come, and the only way to keep up will be with web‐based systems such as the world register of marine species (WoRMS) or with taxon‐specific sites. The scheme shown below is that of Brusca et al. (2016). Brusca et al. divide the Cnidaria into three subphyla: the Anthozoa, Medusozoa (cf. Bouillon 1999), and the parasitic Myxozoa. The Anthozoa are devoid of the medusa phase, and in the Medusozoa the medusa phase is an important phase of the life cycle. The Anthozoa include one class, the Anthozoa, and two subclasses: the Hexacorallia and Octocorallia. All are benthic forms, with the anemones, black corals, and stony corals included in the subclass Hexacorallia, and the gorgonians, soft corals, sea pens and sea pansies and organ pipe corals included in the subclass Octocorallia.

The Medusozoa include the taxa of main concern, the free‐swimming pelagic jellies. Five classes of Medusozoa have been identified, of which three, the Cubozoa (cubomedusae), Scyphozoa (the large jellyfishes), and the Hydrozoa (hydroids and hydromedusae), will garner the most attention in the following pages. Though anthozoans are major players in the benthos of all oceanic systems, our chief interest here is their structural relatedness to the free‐swimming jellies. The similarities between polyps and medusae are best visualized by turning a medusa upside down (Figure 3.1). It is easy to see that both polyps and medusae may be characterized as having a central gut (a gastrovascular cavity or stomodeum) surrounded by a row of radially arranged tentacles.

Figure 3.1 Tissue layer homologies in cnidarians. (a) A hydrozoan polyp; (b) an anthozoan polyp; (c) a hydromedusan medusa, shown upside down for similar orientation. The outer tissue layer is ectodermal (epidermis); the inner tissue layer is entodermal (gastrodermis); the middle tissue layer is mesenchyme/mesoglea.

Phylum Cnidaria

      Subphylum Medusozoa

       Class Hydrozoa

       Subclass HydroidolinaOrder Athecata (Anthomedusae or Anthoathecata)Suborder Capitata (Porpita, Millepora, Coryne)Suborder Aplanulata (Tubularia, Corymorpha)Suborder Filifera (Stylaster, Bouganvillia)Order Thecata (Leptomedusae or Leptothecata) (Aequorea, Campanularia)Order SiphonophoraSuborder – Cystonectae (Physalia)Suborder – Calycophorae (Muggiaea)Suborder – Physonectae (Agalma)

       Subclass TrachylinaOrder LimnomedusaeOrder TrachymedusaeOrder Narcomedusae

      Class Cubozoa – Sea wasps and box jellies

       Order Chirodropida (Chyronex)

       Order Carybdeida (Carybdea)

      Class Scyphozoa – “true jellyfish”

       Order – Coronatae – coronate medusae, common in deep‐sea – Periphylla

       Order – Semaeostomae – most typical large jellyfish – e.g. Aurelia

       Order – Rhizostomae – cannonball jellies

      Class Staurozoa – Stalked jellyfish. Small sessile individuals – polypoid (Halyclistis)

      Class Polypoidozoa – Intracellular parasites of developing fish eggs. Monospecific.

      Subphylum Anthozoa

       Class Anthozoa

       Subclass Octocorallia – three orders and six suborders, includes soft corals, gorgonians, sea pens, and sea pansies.

       Subclass Hexacorallia – six orders, includes the true sea anemones, stony corals, black corals, and tube‐dwelling anemones.

      Subphylum Myxozoa

      Intracellular parasites of marine and freshwater vertebrate ectotherms, annelids, bryozoa, and sipunculids. Two classes and two orders. About 2200 species.

Open‐ocean species are often not as well understood or well described as nearshore species because of insufficient access to specimens. This is particularly true of the Cnidaria. Taxonomic keys and good drawings of species are limited, and many of the best treatments of cnidarian groups are quite old. For decades, the oceanographer’s gold standard for identifying medusae resided in the works of two authors, F. S. Russell (Medusae of the British Isles, volumes I and II) and P. L. Kramp (“The Hydromedusae

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